Synthesis and characterization of semiconductor nanoparticles and hybrid nanocomposites for optoelectronic applications

Interest in nanoparticles and nanocomposites of hybrid organic/inorganic materials has increased considerably over the last decade, an interest that has been fueled by novel and exciting potential applications of these materials as electronic devices such as organic light emitting diodes, solar cells and sensors. Group IV semiconductors as well as compounds formed with group II-VI elements are among the most used inorganic materials in this field, mainly because their physical properties can be significantly altered at the nanoscale. The focus of this thesis is on the realization and the study of size-dependent modification of the electronic properties of two classes of semiconductors, germanium (Ge) and cadmium sulfide (CdS). An investigation of the physical properties of Ge nanocrystals prepared by a consolidated method, so as to achieve nanostructures on silicon dioxide substrates, is presented. Concerning the synthesis of CdS, a rapid and inexpensive method that avoids the mixing of toxic reagents and solvents has been developed for the preparation of colloidal nanoparticles and (CdS)nanoparticle/polymer nanocomposites. In particular the CdS based nanocomposites were realized by two main classes of synthesis: ex- and in-situ. With respect to the former class, the nanocrystals were synthesized in a solvent, then extracted and subsequently combined with a polymer. In the case of the latter class, the nanocrystals were produced directly in the polymer matrix. In both cases the nanoparticles were synthesized by adopting the method of the thermolysis of a single source precursor. Beyond the synthesis, several characterization techniques have been employed to study the chemico-physical, optical and structural properties of both the Ge and CdS nanoparticles and nanocomposites. These include Scanning Tunnel Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS), Nuclear Magnetic Resonance (NMR), Fourier Transform-Infrared Spectroscopy (FT-IR), UV-Vis absorption and photoluminescence (PL) spectroscopy, Atomic Force Microscopy(AFM), Transmission Electron Microscopy (TEM), Wide Angle X-ray Scattering (WAXS) and Grazing Incidence x-ray Diffraction (GID). This thesis may be divided into four main areas of study: 1)With the aim of investigating quantum size effects in Ge nanoparticles, high density Ge nanoparticle samples have been realized via a method of high temperature vacuum evaporation onto SiO2 substrates. By means of electrochemical photocurrent measurements, (a) the ability of the Ge nanoparticles to generate photocurrents in the near ultraviolet and visible spectral ranges and (b) the strong dependence of the photocurrent features on the Ge nanoparticle size are demonstrated. 2) Concerning the II-VI compounds, CdS nanoparticles have been fabricated by thermolysis of a Cd precursor in octadecene (ODE), an ex-situ method. Sample preparation under varying conditions-precursor concentration, temperature, and annealing time and rate was carried out. Some examples of dispersion of nanoparticles in polymers are also presented. 3)In accordance with an in-situ method,CdS nanoparticle-based nanocomposites have been prepared by thermolysis of a Cd precursor in different polymeric matrices,firstly employing a dielectric polymer (e.g. Topas, Tp) and then later conductive polymers like poly(N-vinylcarbazole) (PVK) and poly(3-hexylthiophene) (P3HT). Success in the synthesis of CdS nanoparticles in the 2-5 nm (diameter) range for all polymers employed is demonstrated. Particle size is enhanced by increasing the annealing temperature. 4)Electrical properties of nanocomposites prepared via both methods have been investigated by realizing simple stack devices, e.g. ITO-CdS/polymer-LiF-Al (ITO = indium tin oxide substrate). Such devices were studied by performing current-voltage, electroluminescence and photocurrent measurements.It is deduced from different experiments that the synthesis method adopted can be extended to several polymer producing nanocomposites with improved electro-optical properties.